Thermal field emission cathodes are used in devices such as scanning electron microscopes, transmission electron microscopes, semiconductor inspection systems, and electron beam lithography systems as an electron source. In such devices, an electron source provides electrons, which are then guided into an intense, finely focused beam of electrons having energies within a narrow range. To facilitate formation of such a beam, the electron source should emit a large number of electrons within a narrow energy band. The electrons should be emitted from a small surface area on the source into a narrow cone of emission. Electron sources can be characterized by brightness, which is defined as the electron current divided by the real or virtual product of the emission area and the solid angle through which the electrons are emitted.
Electrons are normally prevented from leaving the atoms at the surface of an object by an energy barrier. The amount of energy required to overcome the energy barrier is known as work function of the surface. A thermionic emission electron source relies primarily on heat to provide the energy to overcome the energy barrier and emit electrons. Thermionic emission sources are not sufficiently bright for use in many applications.
Another type of electron source, a cold field emission source, operates at room temperature and relies on a strong electric field to facilitate the emission of electrons by tunneling through the energy barrier. A field electron source typically includes a narrow tip at which electrons leaves the surface and are ejected into surrounding vacuum. While cold field emission sources are much smaller and brighter than thermionic emission sources, cold field emission sources exhibit instabilities that cause problems in many applications.
Yet another type of electron source is referred to as a Schottky emission cathode or Schottky emitter. Schottky emitters use a coating on a heated emitter tip to reduce its work function. The coating typically comprises a very thin layer, such as a fraction of a monolayer, of an active metal. In Schottky emission mode, Schottky emitter uses a combination of heat and electric field to emit electrons, which appear to radiate from a virtual point source within the tip. With changes to the emitter temperature and electric field, the Schottky emitter will emit in other emission modes or combinations of emission modes. Schottky emitters are very bright and are more stable and easier to handle than cold field emitters. Because of their performance and reliability benefits, Schottky emitters have become a common electron source for modern electron beam systems.
FIG. 1 shows part of a conventional Schottky emitter 12 described in U.S. Pat. No. 5,449,968 to Terui et al. The Schottky emitter 12 is a portion of a thermal emission cathode as the FIG. 2 presents. The Schottky emitter 12 includes a filament 14 that supports and heats an emitter 16; the emitter 16 having an apex 22 from which the electrons are emitted; and a suppressor electrode 51 to prevent electrons emitting from position other than the apex 22. Heating current is supplied to filament 14 through electrodes 61 that are showed in FIG. 2. The Schottky emitter 12 typically operates with apex 22 at a temperature of approximately 1,800K. Emitter 16 is typically made from a single crystal of tungsten oriented in the <100>, <110>, <111>, or <310> direction. Emitter 16 could also be made of other materials, such as molybdenum, iridium, or rhenium. Emitter 16 is coated with a coating material to lower its work function. Such coating materials could include, for example, compounds, such as oxide, nitrides, and carbon compound, of zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium, or lanthanum. For example, coating a (100) surface of tungsten with zirconium and oxygen lowers the work function of the surface from 4.5 eV to 2.8 eV. By reducing the energy required to emit electrons, the coating on the emitter 16 makes it a brighter electron source.
At the high temperatures at which Schottky emitter 12 operates, the coating material tends to evaporate from emitter 16 and must be continually replenished to maintain the low work function at apex 22. A reservoir 28 of the coating material, with a shape as water drop on the emitter, is typically provided to replenish the coating on emitter 16. The material from reservoir 28 diffuses along the surface and through the bulk of emitter 16 toward apex 22, thereby continually replenishing the coating on the apex 22. At the Schottky emitter 12 operating temperature, not only the coating material on the emitter 16 and apex 22 evaporate, the coating material also evaporates directly from the reservoir 28, thus depleting it. The evaporation rate of the coating material in the reservoir increases exponentially with the temperature. Thus, the useful life of the reservoir depends upon the amount of material in the reservoir and its temperature.
When reservoir 28 is depleted, Schottky emitter 12 no longer functions properly, and it is necessary to shut down the electron beam system to replace the emitter 16 or the whole Schottky emitter unit 12. This process is time consuming and is costly. It is desirable, therefore, to extend the life of the reservoir 28 as much as possible, thereby extending the life of the emitter 16. The present invention addresses such a need.